The present invention is related to battery electrodes, battery cells and battery lamination processes. The present invention is especially applicable to a method for making a self-supporting and self-strengthening polymer lithium ion battery of low cost, high efficiency and excellent quality.
In the past decade, the work force has been becoming more and more mobile worldwide, simulating high demands for portable consumer electronics such as cellular phones, laptop computers, PDAs, digital cameras, digital camcorders, etc. In addition, the consumer is demanding more reliable and longer-lasting equipment, both of which are determined by the performance of the battery that fuels the mobile applications.
Rechargeable batteries for powering portable electronics have evolved over three generations, from Ni—Cd to Ni—MH and then to Li-ion battery. Gravimetric energy density for each new generation has increased by 50–100%, by associating with new chemistry, materials and technology. Today, the lithium ion battery still dominates the majority of consumer markets, which projects an impressive 40% compound annual growth rate for at least the next five years. In 1994, Bell Communication Research, Inc introduced the polymer lithium ion battery and patented it (U.S. Pat. No. 5,296,318). Since then numerous U.S. patents have issued on polymer, polymeric or polymer-like electrolyte and lithium ion batteries in an attempt to commercialize and mass produce these batteries. Although the polymer lithium ion battery makes no breakthrough in the chemistry of a lithium ion battery, it does show some advantages: flexibility in its design and fabrication, slim size, and light weight. However, these kinds of polymer lithium ion batteries also have some drawbacks:                (1) They are not a pure polymer battery, but a battery mixed with liquid electrolyte in a polymeric matrix. During its fabrication process, there is no formation of either gel, gelling or gel-like electrolyte in the cells. Typically, a polymer separator membrane is formed by casting a solution, that consists of polymer, solvent and dibutyl phthalate (DBP) as plasticizer, on a glass or a plastic substrate such as Teflon and polyester (PET). The membrane is then laminated between the anodes and cathodes at 120–150° C. for a few minutes. After extraction of DBP at 50–85° C. for more than 45 minutes, the polymer membrane becomes a porous separator and will soak liquid electrolyte in the process, known as “activation”. Liquid electrolyte remains as a secondary phase and as an immobile fluid in the pores of the polymeric matrix. Therefore, these kinds of polymer lithium ion batteries are not free from problems of leakage and corrosion as described in the above-mentioned patents.        (2) Unlike the multi-layer separator used in traditional wet lithium ion batteries, the polymeric membrane separator in a polymer lithium ion battery has no thermal shutdown mechanism that protects the battery from thermal run-away under abusive conditions. The multi-layer separator in the wet lithium ion battery is typically a microporous trilayer membrane with one polyethylene layer sandwiched between two polypropylene layers, so called “polyolefins”. It maintains excellent mechanical strength at elevated temperatures up to its melting point (135–160° C.) where it melts and closes all micropores, resulting in a shutdown of ionic diffusion (internal current). Therefore, when the battery temperature rises to a melting point level in any abusive condition such as overcharge, overdischarge and over-heating, the polyolefin separator would block ionic conductance by closing the micropores. The battery cell would have very high internal impedance, which would not allow current to pass through. The temperature would start to cool down, and the battery would be protected from thermal run-away. However, the separator in the polymer battery is a polyvinylidene fluoride (PVDF)-based membrane, having no mechanical strength at elevated temperatures and no thermal shutdown protection. The liquid electrolyte solution in the polymeric matrix of the polymer battery will not be blocked and will continue to carry current when battery temperature rises. For this reason and from a product safety point of view, the polymer lithium ion battery would not be safer than the wet lithium ion battery; in some cases, it may be just the reverse.        (3) The cell of a polymer lithium ion battery is not self-supported, having little mechanical strength. The battery would easily swell and be deformed, as the outer package is a soft bag with laminated metallized plastic.        (4) The larger internal impedance causes poorer performance at both low temperature and after high temperature storage. Due to electrolyte immobilization in a polymeric membrane, a necessary function of cell construction, safety and dimensional flexibility in a polymer lithium ion battery is a reduction in ion transport rate. This generally results in larger internal impedance than that in a wet lithium ion battery. The consequences of larger internal impedance are deterioration in discharge capability, especially at low temperatures, and recycling efficiency, particularly under conditions of use after elevated temperature storage conditions.        (5) Polymer lithium ion batteries have high production costs due to slow fabrication processes and low yield rate. Productivity of polymer lithium ion batteries is lower than that of wet lithium ion batteries, due to slower processes such as the DBP extraction process above-mentioned and bi-cell stacking process. The lower production yield rate is mainly due to difficulty in control of homogeneity of membrane thickness, electrode loading, adhesion between electrodes and polymer membrane for the large area of laminated electrodes.        
Another approach to make a polymer lithium ion battery proposes to coat the microporous polyolefins separator, prior to the electrodes/separator lamination process, with a layer of a bonding paste comprising gelling polymer having an electrolyte active species. However, in the following assembly processes, the occurrence of delamination or separation of the electrodes from the coated separators is detrimental. This normally results in a low production yield rate, poor quality, and inconsistency of battery performance. The main cause is difficulty in control of the coating separator with a bonding paste that comprises electrolyte solution. Solvent or co-solvents in the electrolyte solution are so volatile that it is almost impossible to handle the coating process in open-air operations to get a high quality layer on the separator. The surface density and gelling degree of paste layers would vary a lot, depending upon the coating speed, formulation of bonding paste, environmental temperature, and the surface area of coating being exposed to atmosphere.
Therefore, there is a need for a new method that is innovative, simple, reliable, highly productive and cost-effective to be used for making a polymer lithium ion battery with high quality and consistent performance.